1. Field of the Invention
The invention relates to a sound field measuring apparatus and a sound field measuring method capable of calculating positional relationship of speakers in real space as output means for forming, for example, a multichannel audio system.
2. Description of the Related Art
In playback systems of video data, musical data and the like, it is relatively easy for users to evaluate realistic sensation or sound quality as good or not good. For example, when a user listen to an orchestral piece, it is preferable that a sound field can be generated, in which the user can sense positions of respective instruments clearly and can recall an image as if a real orchestra performs right before the user in a virtual sound field.
For example, there are a two-channel stereo system which adjusting sound volume of respective signal channels of two channel stereo signal including a L-signal and an R-signal, so that a sound image of a playback sound field is located in an optimum position as a virtual sound image, and outputs signals from two speakers, a three-channel stereo system in which a center speaker is added in the middle of right-and-left two channel speakers, 5.1 channel stereo system in which further rear speakers are added, and the like.
For example, in a multichannel audio system such as the 5.1 channel stereo system, parameters of audio signals outputted from respective speakers are decided so as to reproduce a realistic sound field. For example, the balance of sound volume and sound quality of playback audio at the position where the listener listens vary depending on a so-called listening environment including a structure of a listening room, a user's position with respect to speakers and the like, therefore, there was a problem that the sound field (acousmato) which is actually felt by the listener may be different from the ideal playback sound field created at the time of recording.
The above problem is prominent in a small space such as a small room and in a car. In the interior of the car, the listener's position is limited to the position of a seat in many cases, a distance interval between speakers and the listening position is large. Therefore, time differences of reaching time of audio signals outputted from speakers occur and the balance of the sound field is lost significantly. Particularly, the car interior is in an almost sealed condition, reflection sound and the like are intricately synthesized and reaches the listener, which becomes a factor of confusing the playback sound field in the listening position. Further, in the small room or in the car, positions of installing speakers are limited, when it is difficult to realize speaker positions where output sound from speakers directly reaches ears of the listener, changes of sound quality due to the speaker positions affect deterioration of the playback sound field.
Accordingly, in order to create the playback sound field closed to the original sound field as much as possible according to the listening environment in which the listener actually uses the audio system, appropriate acoustic correction is performed to output audio signals. First, audio characteristics in the listening environment are measured, then, parameters of signal processing to which the acoustic correction is performed are set to an audio output system of the audio set based on the measured result. The audio signals processed according to the set parameters are outputted from speakers, thereby reproducing a good sound field which has been corrected so as to fit into the listening environment. As the acoustic correction, for example, delay time to be given to the audio signals may be corrected according to reaching time from the speakers to the listening position, so that the audio signals of respective channels outputted from speakers reach the listening position of the listener (position of ears) almost at the same time.
As an example of measurement of acoustic characteristics and acoustic correction based on the measurement, the following method using an acoustic correction apparatus disclosed in Patent document 1 is known.
First, a microphone for measurement is arranged at a position of the listener's ears (listening point) in a space in which the audio set is used, namely, in the listening space. Then, a measuring tone is outputted from the speaker, and the measuring tone is collected by the microphone, and distance information between each speaker and the listening position (setting position of the microphone, namely, position of collecting sound) is calculated from characteristics of the collected audio signal. Since reaching time of audio in a space from respective speakers to the listening position can be obtained based on the distance information, the acoustic correction apparatus can set delay time of the audio signal of a the channel corresponding to each speaker by using information of reaching time of respective speakers, so that timings at which audio emitted from respective speakers reach the listening position coincide. Accordingly, to correct reaching time and phase displacement of audio signals until the listening point is called as a time alignment adjustment.
Patent document 1: JP-A-2000-261900